Since spiral information tracks recorded on a disc are apt to be eccentric with respect to a center hole of the disc, means for causing a reproducing means to scan along information tracks, which means is called a tracking servo, has hitherto been required in order to reproduce tracks of such a disc.
Since such tracking servo is well known, a detailed description of the structure thereof is omitted.
FIG. 1 is an explanatory diagram for the principle of a servo system using three beams, which is a system of a tracking servo for an optical disc used for obtaining a reproduced signal by way of a light beam. In FIG. 1, the reference T.sub.1, T.sub.2 and T.sub.3 are information tracks of a disc; 2 is a light beam for reading; and 1 and 3 are auxiliary beams for detecting tracking signal, and the difference between reproduced outputs of the auxiliary beams 1 and 3 is detected using a differential amplifier so as to obtain a tracking signal 4 shown in FIG. 1. In the above, since the information tracks T.sub.1, T.sub.2 and T.sub.3 are moving relative to the light beams 1, 2 and 3 due to the eccentricity of the disc, the tracking signal 4 shown in FIG. 1 is obtained. A range "A" within the tracking signal 4 indicates that the beams 1, 2 and 3 are scanning the track T.sub.2, a range "B" indicates that a portion between the tracks T.sub.1 and T.sub.2 is being scanned.
FIG. 2 is a structural diagram of the tracking servo. The reference 10 is a disc; 11 is a motor for rotating the disc 10; 12 is a reading lens; 13 is a light source used so that the disc 10 is irradiated by auxiliary beams 1, 2, 3 for tracking as shown in FIG. 1 using a diffraction grating 14. The reference 15 is a beam splitter; 16 is a tracking mirror, and these elements are used for controlling the position of the light beam so that the reading light beam, which is focussed on the disc 10 by way of the reading lens 12, is always on a track. The references 17 and 18 are photo detectors for receiving reproduced outputs of the auxiliary beams 1 and 3 shown in FIG. 1, and the tracking signal 4 of FIG. 1 is obtained at an output of a differential amplifier 19. The reference 20 is an equalizer which is a filter for correcting the characteristic of the tracking servo system. The reference 21 is a D.C. amplifier which supplies a current corresponding to the output from the equalizer 20 so as to effect tracking control wherein the deflecting angle of the tracking mirror 16 is controlled.
Assuming that the rotational speed of the disc 10 is 1800 rpm, the eccentricity of the disc 10 has a harmonic component of a fundamental wave of 30 Hz, and therefore, the tracking mirror 16 has to be operated so as to scan an information track of the disc 10 by the reading light beam 2 of FIG. 1 by tracking the eccentricity. As a result, at least the band of tracking servo is required to cover all the harmonic components of the above-mentioned eccentricity. For instance, assuming that 30 Hz component of eccentricity is 100 micron and 1 kHz component is 2 micron, the reading light beam 2 has to follow the information track of the disc 10 with an accuracy of .+-.0.1 micron. Therefore, the gain of the tracking servo at 30 Hz is required to be 100/0.2=500, and the gain at 1 kHz is required to be 2/0.2=10 (wherein 0.2 means .+-.0.1 micron). In this way the tracking servo band width and gain of the same are selected.
FIG. 3 illustrates a block diagram of the tracking servo system described with reference to FIG. 2. The reference R is a signal indicative of the position of an information track of the disc; the reference C, a signal indicative of the position of the reading light beam, and the difference between these signals becomes an error signal E, and therefore, the tracking servo system can be expressed as a servo system in which the tracking mirror G3 is controlled through the equalizer G1 and the D.C. amplifier G2. In the above, G1, G2 and G3 respectively indicate transfer functions. The reference SW is a switch which is arranged to cut the signal C temporarily for the description of malfunction.
FIG. 4a shows a frequency response of the above-mentioned equalizer and tracking mirror G1, G3 and it is assumed that the frequency response of the DC amplifier G2 is flat for simplicity. FIG. 4b shows open-loop characteristic of the block diagram shown in FIG. 3, and the reference K1 indicates the open-loop gain.
While the conventional tracking servo has been described in the above, when a defect, such as a scratch at the information track(s) of the disc, exits, the signal C of FIG. 3 indicative of the position of the light beam cannot be detected, and this phenomenon can be considered as the switch SW in the block diagram of the tracking servo (see FIG. 3) is cutoff for a given period of time. Such a phenomenon can be actualized by painting ink by a felt pen on a disc, and when the size of a defect becomes larger than a given size, the tracking servo malfunctions resulting in the occurrence of track jumping and locked group. Analyzing this phenomenon, the following points have been made clear.
(1) When gain crossover frequency in the tracking servo is made high, stability against disturbance, such as vibrations applied to a disc player, is satisfactory although malfunction is apt to occur with a small defect.
(2) When gain crossover frequency is made low within a range where disc eccentricity tracking is available, malfunctions due to defects are few and far between. However, stability against disturbance, such as vibrations applied to a disc player, is unsatisfactory.
This can be explained with reference to FIG. 5 which corresponds to FIG. 1, and center lines of information tracks are indicated at the references T1, T2 and T3. Regions A and B in FIG. 1 correspond to regions A and B in FIG. 5. Let us assume that the reading light beam 2 is scanning the track T2 in a direction of an arrow 5 by way of the tracking servo. The reference 6 indicates defects on the track, and at this defect 6 portion the switch SW in FIG. 3 is cutoff resulting in the occurrence of the tracking servo causing jumping in a direction of an arrow 7 or 8 depending on the characteristic of the servo. In the case that the band width of the open-loop characteristic expressed in terms of G1xG2xG3 of FIG. 3 is wide, jumping results suddenly as shown by the arrow 7 so as to jump out of the region A with time t1. On the other hand, when the band width of the open-loop characteristic is narrow, jumping results slowly as shown by the arrow 8 so as to jump out of the region A with time t2. Namely, since the error signal E is amplified by G1, G2 and G3, the time for jumping out of the region A is determined by the frequency response of G1, G2 and G3.
The region A, as will be understood from FIG. 1, is a dynamic range of the tracking servo, in which the reading beam can be held on the track T2, while the region B is an unstable range in which the polarity of the tracking servo is inverted. Therefore, if the size of the defect 6 is larger than t1, t2, the reading beam is off the track as indicated by the arrows 7 and 8, respectively. If the size of the defect 6 is smaller than the same, then the reading beam is again held on T2.
In this way, while the occurrence of malfuntion due to track defect can be reduced if the band width of the open-loop characteristic of the tracking servo is made narrow and the gain crossover frequency is made low, the operation of the tracking servo becomes unstable in conection with a disc with large eccentricity, while the tracking servo malfunctions when disturbance, such as vibrations, is applied to the disc player.